Sectoral antennas are those which are designed to transmit or receive signals over a selected sector. A recent phenomenon which has highlighted the usefulness of sectoral antennas is the development of cellular telephones. Sectoral antennas are ideally suited for use in a given cellular telephone operating area.
Sectoral antennas have to date generally been adjusted at the factory to obtain the desired pattern. Normally these antennas have not been field adjustable or may be adjustable only with substantial difficulty after installation.
To date only corner reflectors have in some cases been made field adjustable. The relative positions of the sides of the reflector would be changed about the angle by adjusting the dimensions of the sides or changing the angle. These adjustment methods are effective but have been found to be generally undesirable as a result of physical problems. Changes in wind loading on the support structure have created problems with these types of adjustments and, as well, the mechanisms required to change the angle of the corner have been overly complicated for field use.
A substantial operating problem with prior art adjustable antennas has been a difficulty in maintaining an acceptable VSWR over wide band widths. With the corner reflectors adjustment has tended to reduce the useable band width of the antennas. A significant need has arisen for field-adjustable sectoral antennas which will maintain an acceptable VSWR over wide band widths. With the very rapid increase in the use of cellular telephones, the need to be able to adjust existing antennas to meet rapidly changing conditions became evident. No practical means was available for dealing with the problem.
Against this background the present invention provides a field adjustable sectoral antenna which is easily adjusted in the field to provide a wide range of patterns in the horizontal plane, while maintaining an acceptable VSWR over the required band width.